Dropping security_training_providers table

What does this MR do and why?

Closes: #478629 (closed)

This is the final MR in the removal chain. In %19.1 we removed the rails model (!238533 (merged)), and this MR removes the table in %19.2.

From the Danger comment:

1. How the data could be recovered in the event of an incident.
   1. It is noted in the code block below. Those rows would need to be re-added either with another migration or by re-adding the seeder script removed in !238533 (merged)
2. The approximate number of records being affected.
   1. There are three records.
3. A description of the user experience impact.
   1. There should not be a user experience impact. !238533 (merged) removed the AR model entirely, so there should be no use of that table anymore.

The static data that is removed on SaaS is:

gitlabhq_dblab=# select * from security_training_providers;
-[ RECORD 1 ]-----------------------------------------------------------------------------------------------------------
id          | 1
name        | Kontra
description | Kontra Application Security provides interactive developer security education that                        +
            |                   enables engineers to quickly learn security best practices                              +
            |                   and fix issues in their code by analysing real-world software security vulnerabilities.
url         | https://application.security/api/webhook/gitlab/exercises/search
logo_url    | 
created_at  | 2022-03-10 13:20:20.524696+00
updated_at  | 2022-03-10 13:20:20.524696+00
-[ RECORD 2 ]-----------------------------------------------------------------------------------------------------------
id          | 2
name        | Secure Code Warrior
description | Resolve vulnerabilities faster and confidently with highly relevant and bite-sized secure coding learning.
url         | https://integration-api.securecodewarrior.com/api/v1/trial
logo_url    | 
created_at  | 2022-03-10 13:20:20.524696+00
updated_at  | 2022-03-10 13:20:20.524696+00
-[ RECORD 3 ]-----------------------------------------------------------------------------------------------------------
id          | 34
name        | SecureFlag
description | Get remediation advice with example code and recommended hands-on labs in a fully                         +
            | interactive virtualized environment.
url         | https://knowledge-base-api.secureflag.com/gitlab
logo_url    | 
created_at  | 2023-05-10 17:26:02.182658+00
updated_at  | 2023-05-10 17:26:02.182658+00